The present invention relates to a ballast circuit for fluorescent lamps. More particularly, the present invention relates to a highly efficient ballast circuit adaptive to changes in the input voltages.
Fluorescent lamps are becoming increasingly popular for use in homes or offices because of their high operating efficiency as compared to incandescent lamps. Indeed, fluorescent lamps emit light at several times the efficiency of a typical incandescent lamp, and do not generate as much heat as a typical incandescent bulb, thereby conserving radiant energy and eliminating excess heat output.
A typical fluorescent lamp is constructed from a glass tube which contains two electrodes at opposite ends, a coating of powdered phosphor covering the interior of the tube, and small amounts of mercury. The major components of a fluorescent lamp are the bulb, electrodes, fill gas, phosphor coating and a base used to support the external conductors of the electrodes. When energized, the electrodes produce a large potential between which free electrons initiate an arc. The arc generates some visible radiation, but mostly ultraviolet radiation, which in turn excites the phosphor coating causing it to emit light. In this process, the fluorescent effect is caused by the excited mercury vaporized in the arc.
Larger fluorescent systems are well known and have involved larger scale, well known illumination electronics which have been optimized to one degree or another over the years. A variety of techniques are developed for optimizing these larger fluorescent systems such as to eliminate flicker; provide light which is suitable for reading and working, and provide adequate lighting in large industrial facilities, etc.
As fluorescent lighting has gained acceptance for increasingly less formal and consequently smaller uses, the sizes of the fluorescent lamps and their systems have become smaller. The reduced scale of the fluorescent lights cannot generally be accomplished with a simple size reduction in the circuit elements. Care must be taken to re-design the operating electronics to insure first that a size reduction is achieved, and second that the higher power efficiency is preserved in the smaller size lamps. This is especially important because it is the power efficiency which generated the interest in the use of fluorescent lighting initially.
In a conventional fluorescent lamp design, a ballast circuit is required for providing a high and constant voltage to operate the fluorescent lamp. In addition to the high voltage requirement, the ballast circuit is required to maintain a constant current flowing to the lamp. It is well known in the art to use a close-loop feedback design to control the ballast circuit for providing a constant high output voltage and current.
FIG. 1 shows a conventional design of a ballast circuit. In the ballast circuit as shown, an input voltage Vi is provided to a rectifier 1. The rectified input voltage Vir is then fed into a voltage conversion circuit 2 for generating an output voltage Vo higher than the rectified input voltage Vir. The voltage inverter 3 then converts the high output voltage Vo to a high frequency output voltage Vof for the fluorescent lamp operation.
FIG. 2 shows a voltage conversion circuit 2 of the conventional close-loop feedback design. The voltage conversion circuit 2 comprises a voltage converter 24 for amplifying the rectified input voltage Vir to the high output voltage Vo; and a gain controller 20 for controlling the voltage gain for the voltage converter 24. In the conventional close-loop feedback design, the voltage gain of the voltage converter 24 is inversely proportional to the voltage difference between the output voltage Vo and a reference voltage Vref to ensure the output voltage Vo to be constant notwithstanding changes in the rectified input voltage Vir. Usually, the output voltage Vo is designed to be around 180 to 250 volts when the input voltage is 110 volts. However, there are two problems facing the conventional close loop feedback ballast circuit design.
First, when the input voltage is low, the conventional ballast circuit needs to perform a large amount of energy conversion to maintain a constant high voltage. However, the more energy conversion is performed, the more self-loss is from the ballast circuit. Therefore, the conventional close loop feedback design suffers from low operating efficiency when the input voltage is too low.
The second problem facing the conventional design is high output sensitivity to the electronic components used. In the conventional ballast circuit, the high output voltage is typically controlled by resister sampling and stabilizing power source technologies. These techniques are both highly sensitive to component accuracies. Therefore, higher grade components are needed for the conventional design and thereby the cost of volume manufacturing of the conventional ballast circuits is prohibitive.